The Australian Bureau of Statistics is warning people not to post pictures online that reveal the unique barcode on their same-sex marriage survey forms amid concerns it could jeopardise individual results.

Marriage postal surveys have begun to arrive in Australian mailboxes, but people are already taking to social media to upload images of their forms, some even posting photos with their personal details and unique barcodes in clear view.

The barcode, which appears in the bottom right corner of the survey form, is used to link a specific form to an eligible voting Australian.

The purpose of it is to ensure no one can respond to the survey more than once.

While the ABS has confirmed a unique barcode will not be counted more than once, the independent statistical agency has warned those who share their forms online are at heightened risk of having their vote tampered with.

Before sharing a snap of your survey form on Instagram, Twitter or Facebook, consider this possible scenario:

You share a photo of your survey online, but before you have sent your completed form in the post, somebody else has replicated your identical barcode based on the image you have shared.

They have created a new form, with your barcode and cast their vote.

Under ABS guidelines, the barcode will be used to register the arrival of a form and is anonymous, so if the replicated survey arrives in the post first their vote be counted not yours.

An ABS spokesman said while there wasn't a genuine threat of widespread fraud he urged people to take extra precautions to protect their individual vote.

"Posting personal information of any sort online is generally ill-advised and this is no exception," he said.

"We're instructing people to follow the instructions that come with every survey form and return the form with their preference. If people follow the instructions, and they take care their of own privacy, we expect the survey process will be as effective."

He declined to speculate on whether there was a risk campaigners from either side of the debate would collect barcodes and illegally votr.

"We've got no evidence of that so far so it would be inappropriate to comment," he said.

Australia Post is sending out 600,000 survey forms per day in the coming weeks to reach the more than 16 million people on the electoral roll.